Fluorescence excited in a thunderstorm atmosphere by relativistic runaway electron avalanches
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Fluorescence Excited in a Thunderstorm Atmosphere by Relativistic Runaway Electron Avalanches L. P. Babich* and E. I. Bochkov Russian Federal Nuclear Center, All-Russian Research Institute of Experimental Physics, Sarov, Nizhny Novgorod oblast, 607188 Russia *e-mail: [email protected] Received May 5, 2016
Abstract—The spectrum and spatiotemporal evolution of the fluorescence of an atmospheric discharge developing in the regime of relativistic runaway electron avalanche (RREA) generation have been calculated without involving the relativistic feedback. The discharges generating narrow bipolar pulses, along with the discharges responsible for terrestrial gamma-ray flashes, are shown to be relatively dark. Nevertheless, the fluorescence excited by a discharge involving RREAs can be recorded with cameras used to record high-altitude optical phenomena. A possible connection between a certain class of optical phenomena observed at the tops of thunderclouds and RREA emission is pointed out. DOI: 10.1134/S1063776117050090
1. INTRODUCTION Very hard gamma-ray flashes of atmospheric origin [1–4] called terrestrial gamma-ray flashes (TGFs) have been recorded since 1993. The TGF duration ranges from several tens of microseconds to several milliseconds [5]. Among all of the natural high-energy phenomena occurring in the terrestrial atmosphere, TGFs generate the hardest photon spectrum extending to energies ~100 MeV [4]. Despite their atmospheric origin, TGFs are mostly recorded from artificial Earth satellites: CGRO (Compton Gamma-Ray Observatory) [1], RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) [2], FGRST (Fermi Gamma-Ray Space Telescope) [3], and AGILE (Astro-rivelatore Gamma a Immaginin Leggero) [4]. A TGR initiated by a triggered lightning was also observed from the ground [6]. A mechanism of relativistic runaway electron avalanches (RREAs) capable of developing in the relatively weak but extended electric fields of thunderclouds was proposed in 1992 [7]. It has been assumed almost since the first detection of TGFs that they are attributable to the bremsstrahlung of high-energy electrons of atmospheric origin [8]. This was confirmed by Monte Carlo numerical simulations of the transport of gamma-ray photons generated by RREAs in the Earth’s atmosphere (see, e.g., [9, 10]). The sources of TGFs were shown in [9] to be located at altitudes in the range from 15 to 21 km. It follows from the numerical simulations of the transport of gammaray emission into near space that the best agreement of
the calculated gamma-ray spectrum at the altitude of the RHESSI orbit with the measured spectrum is achieved if the sources of TGFs were at altitudes of 15–20 km [9]. The RREA development is accompanied by the production of a large number of electron–ion pairs through the ionization of air molecules [11–16] and, consequently, a growth of the electric conductivity inside the thundercloud, which must lead to a weakening of the thunderstorm electric field. The generation of electromagnetic pulses (EMPs)
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